130 calibrated Spot-Spitters® (Roberts Irrigation Products Inc., San Marcos, CA) located in each container. Reference evapotranspi- ration (ET) was determined gravimetrically in selected plants, and used to estimate irrigation volumes to apply. Sufficient solu- tion was applied to produce a target leaching fraction of 30%. Leachates were collected at discrete times over the experimen- tal period in three plants per cultivar × salt level combination, and analyzed for pH, electrical conductivity (EC) and Chloride (Cl) concentration (according to Adriano and Doner 1982). From October 10–15, the plants were evaluated for growth (height, width) and leaf chlorophyll index (SPAD 502, Mi- nolta, Japan). Plant growth index was calculated as the sum of plant height plus width 1, plus perpendicular width 2, and the total divided by 3. In addition, plant salt burn ratings were sub- jectively assessed by two judges, using a scale of 0 to 5 (0 = no salt damage, 5 = salt burn damage on all leaves). Thereaf- ter the plants were destructively harvested, separated by or- gans (stems, leaves, and roots) and dried at 70ºC (158ºF) until constant weight. Leaf tissues were ground to pass a 40-mesh screen and subjected to a full nutrient analysis (macro- and micronutrients), by using standard ICP spectroscopy proce- dures (conducted at the Soil Testing and Plant Analysis Labora- tory of Louisiana State University, Baton Rouge, LA). Leaf Cl concentrations were determined according to Gilliam (1971). Plant yield and quality responses to treatments were analyzed by ANOVA and regression procedures using SAS software (ver- sion 9.1, SAS Institute, Cary, NC). For regression procedures, the dry weights and growth index data were converted to relative values, which allowed the evaluation of the cultivars responses to salt treatment without the confounding effect of the absolute dry weight and growth differences inherent to each cultivar. This was accomplished by identifying the plant with the high- est dry mass within each cultivar (across all salt treatments) and assigning it a value of 100, which was then used to calculate the relative value for the rest of the plants within that cultivar. RESULTS AND DISCUSSION The addition of NaCl salt concentrations higher than 6 mM to the irrigation water produced average leachate EC and Cl con- centrations in excess of 2.5 dS/m1 (mmhos/cm) and 20 mM (710 ppm), respectively (Figure 1). These values, representative of those being experienced by the roots, have been reported to sig- nificantly and negatively affect the growth of ornamental trees and large shrubs like Arbutus unedo (strawberry tree), Abelia grandiflora (glossy abelia), Cotoneaster congestus (Pyrenees cotoneaster), Mahonia aquifolium (Oregon grape holly), Liri- odendron tulipifera (tulip tree); Feijoia sellowiana (pineapple guava), as well as an unnamed Lagerstroemia indica (common crapemyrtle) cultivar (Francois and Clark 1978; Francois 1982). Before presenting the salinity response data of the evaluated cultivars, it should be noted that throughout the study it was read- ily apparent, the inherent vigor of the two interspecific hybrids ‘Natchez’ and ‘Basham’s Party Pink’ visibly dominated over the biomass and growth responses of ‘Pink Lace.’ In southern U.S. landscapes, ‘Pink Lace’ is considered a “medium” grower (Egolf and Andrick 1978), reaching mature heights of 3–4.5 m (10–15 ft), contrasting to the 6–9 m (20–30 ft) mature heights of ‘Natchez’ and ‘Basham’s Party Pink’ (Egolf and Andrick 1978; Byers 1997; Dirr 1998). Within the two interspecific hy- ©2009 International Society of Arboriculture Cabrera: Salinity Tolerance of Crapemyrtles Figure 1. Average electrical conductivity (EC) and chloride con- centration (C) in leachates collected from containerized plants of three crapemyrtle (Lagerstroemia) cultivars irrigated with in- creasing levels of NaCl salinity. Data points are means ± standard errors of 27 leachate samples. brids the ‘Natchez’ plants had higher leaf areas, total dry weights and growth indices, but ‘Basham’s Party Pink’ had the highest root dry weights and lower shoot to root ratios (data not shown). It should be noted that ‘Basham’s Party Pink’ can reach larger heights and widths than ‘Natchez’ on the milder parts of the southern United States and Gulf of Mexico regions, namely USDA Hardiness Zone 9a, but whose growth and performance significantly declines in colder zones (Byers 1997), common- ly freezing to the ground. The present study was conducted in Dallas, TX, which is classified as a colder Hardiness Zone 8a. If actual plant biomass and growth measurements are used, all the measured response variables show an interaction of culti- var selection and salinity levels (data not shown), with inherently large growth differences likely masking the relative response of the cultivars to increasing salt stress. Therefore, with the objec- tive of having a better assessment of the relative salinity tolerance of the crapemyrtle cultivars without involving their genetically- determined growth differences, the use of relative values for dry weight and plant growth response variables was considered as a suitable method. This comparative approach was based on the clas- sical evaluation of relative yield responses of field (agronomic and horticultural) crops to increasing salinity conditions (Maas 1990). The interactive effects of cultivar by salt concentration ob- served when using actual values (data not shown) were ef- fectively lost for total plant dry weight, leaf area and top dry weights when these were expressed on a relative basis (Figure 2, Table 1), and the plants from all three cultivars responded in- distinctively, and negatively, to the salinity main effect. While Figure 2a shows only relative total plant dry weight as a func- tion of salt stress, relative leaf area and relative top dry weights (not shown) were similar in their response. Interestingly, while relative root dry weight (Figure 2b) and relative plant growth index (Figure 2d) also responded negatively to the increasing
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